Hi Mark,
Using a single duplicated image doesn't give you any advantage in terms of dynamic range (the information was either captured (and in contained within the file) or it was wasn't (in which case no about of pushing and prodding and rain dancing is going to put it there) - but I agree it CAN make it easier to process the shot (easier to blend an opaque mask gradually than it is to try and select a given area (with appropriate feathering) and process it seperately with agressive levels / curves layers).
Personally, I'm using the GND tool a lot more these days - it's just so darn quick.